The proposed project is a fluorescence study of molecular motion in living nerve and muscle cells before, during, and after nerve-muscle contact and synaptogenesis. Emphasis is placed on the following phenomena: (1) The lateral motion and possible nerve-induced lateral redistribution of acetylcholine receptors in the plasma membrane of chick and rat muscle fibers. Biological specimens will be mainly primarily co-cultures of developing myotubes with neural cells, as well as non-neural myotube cultures, fetal muscle, and adult denervated muscle. Acetylcholine receptors will be specifically marked for visualization on living cells by fluorescence-labeled alpha-neurotoxin. (2) Membrane and axoplasmic flow in developing neurons in cell culture as they differentiate, extend processes, and innervate muscle cells. The optical techniques to be employed allow continous, non-perturbing observation of subportions of single viable cells. Fast lateral molecular motion will be measured by novel variations of the recently developed fluorescence photobleaching recovery technique. Slow molecular redistributions will be recorded by time-sequence fluorescence photography. Synaptic function in cell culture will be checked by microelectrophysiological techniques. The project aims toward answering specific, well-defined questions in the molecular aspects of how cells of the neuromuscular system develop, form membrane specializations, and recognize and contact each other.